Add metrics support for Azure & GCS Gateway (#8954)

We added support for caching and S3 related metrics in #8591. As
a continuation, it would be helpful to add support for Azure & GCS
gateway related metrics as well.

cmd/gateway/azure/gateway-azure.go

@@ -146,7 +146,14 @@ func (g *Azure) NewGatewayLayer(creds auth.Credentials) (minio.ObjectLayer, erro
 		return &azureObjects{}, err
 	}
 
-	httpClient := &http.Client{Transport: minio.NewCustomHTTPTransport()}
+	metrics := minio.NewMetrics()
+
+	t := &minio.MetricsTransport{
+		Transport: minio.NewCustomHTTPTransport(),
+		Metrics:   metrics,
+	}
+
+	httpClient := &http.Client{Transport: t}
 	userAgent := fmt.Sprintf("APN/1.0 MinIO/1.0 MinIO/%s", minio.Version)
 
 	pipeline := azblob.NewPipeline(credential, azblob.PipelineOptions{
@@ -168,6 +175,7 @@ func (g *Azure) NewGatewayLayer(creds auth.Credentials) (minio.ObjectLayer, erro
 		endpoint:   endpointURL.String(),
 		httpClient: httpClient,
 		client:     client,
+		metrics:    metrics,
 	}, nil
 }
 
@@ -357,6 +365,7 @@ type azureObjects struct {
 	minio.GatewayUnsupported
 	endpoint   string
 	httpClient *http.Client
+	metrics    *minio.Metrics
 	client     azblob.ServiceURL // Azure sdk client
 }
 
@@ -460,6 +469,11 @@ func parseAzurePart(metaPartFileName, prefix string) (partID int, err error) {
 	return
 }
 
+// GetMetrics returns this gateway's metrics
+func (a *azureObjects) GetMetrics(ctx context.Context) (*minio.Metrics, error) {
+	return a.metrics, nil
+}
+
 // Shutdown - save any gateway metadata to disk
 // if necessary and reload upon next restart.
 func (a *azureObjects) Shutdown(ctx context.Context) error {